Metal containing minerals, especially metal sulphides, may be oxidised using specific types of micro-organism, especially bacteria in a bioextraction, particularly a bio-oxidation process. Oxidation of sulphide minerals may be used to put the metals into solution, from for example iron, copper, zinc, nickel and cobalt sulphides, or to release precious metals, such as gold, silver and platinum, encapsulated in metal sulphides, particularly in refractory ores. The process is known as bacterial oxidation, bioextraction, bio-oxidation, bioleaching, or bacterial leaching. In this respect, pyrite, an iron sulphide, and arsenopyrite, an iron-arsenic sulphide are the most common minerals occluding gold in the so-called refractory gold ores and treatment of such materials by microorganisms may therefore assist in liberation of precious metals from refractory ores containing them.
Certain kinds of bacteria used in such processes have been documented and include the mesophiles, Thiobacillus ferrooxidans, Thiobacillus thiooxidans, Leptospirillum ferrooxidans, moderate thermophiles, and thermophiles such as Sulpholobus. 
The above processes are the subject of an extensive literature encompassing papers and patents. However, conventional methods of bacterial oxidation involve oxidation in agitated stirred tanks into which air is introduced or in heaps using rocks with a size of 6 mm or greater in which fine particles may have been agglomerated or heaps using pelletised fine material. An acidic bacteria containing liquor is developed and typically used to irrigate the heap and facilitate metal extraction. The heap leaching technique has been used for many years and Brierley et al have patented a process (U.S. Pat. No. 5,246,486) for a two stage agglomeration process for the recovery of gold from sulphide minerals in heaps where the mineral particles are of large size and not slurried.
Van Aswegen in the article “The Genmin Experience”, has described bacterial oxidation in tanks made of stainless steel or other lined steel which contain a slurry of concentrate and water. The tanks are typically 9 meters high and 9 meters diameter and agitated with axial flow impellers which are driven by large motors. Compressed air is piped into the tank and sheared by the impellers Into a fine dispersion of bubbles to maintain a desired dissolved oxygen concentration in the slurry. A variety of impeller types other than axial flow may be used such as turbines.
The present applicant has found that such methods suffer from a number of disadvantages. Primary among these is the high power consumption and high capital cost encountered in employing bacterial oxidation reactors agitated by impellers.
Although proposals have been made to employ diffusion means to maintain particles in suspension, notably in Envirotech U.S. Pat. Nos. 4,728,082, 4,732,608, 4,968,008, 4,974,816 and 5,007,620, it is uniformly recognised that there is a need to provide some mechanical means, for example in the form of a rake to ensure that significant quantities of solids are not deposited on the floor of the reactor vessel, reducing reactor efficiency.